Turbines are used, for example, in aircraft engines and as industrial gas turbines. When using turbines, their efficiency is an important characteristic and is therefore determined at a high cost under operating conditions during the development of devices in which the turbines are to be used. For this purpose, the temperatures and pressures at the turbine intake and turbine outlet, which are averaged radially and over the circumference, are primarily measured. The efficiency is inferred from these measurements and from further measured variables via an overall model of the propulsion unit or the gas turbine. Inhomogeneities in the temperature and pressure fields and further effects, some of which are difficult to estimate, typically result in uncertainties of ±2% or more. The power output by the turbine to the compressor via the shaft, which may be measured via the torque applied to the shaft, represents an additional independent variable which may be used to improve the efficiency determination.
In order to significantly improve the efficiency determination, the torque must be determined with a precision of ±0.5% or better. As much as possible, the measurement is to require no modifications or only slight modifications to the shaft and to the engine and/or the gas turbine and is to be performable rapidly and cost-effectively overall.
To determine torques on shafts, typically either the torsion angle is measured over a finite part of the shaft or the torsional strain is measured locally on the shaft surface. In both cases, the torque relates to the measured variable via the shear modulus and the geometry of the shaft.
Different methods have been described in the related art for measuring the torsion angle. In the simplest case, ring gears are attached to both ends of the shaft, which then produce alternating signals, whose relative phase position changes if the shaft is subjected to torsion in non-rotating optical or electrical (e.g., inductive) sensors.
Torsional strains are usually measured using strain gauges or using magnetostrictive metal films, the latter changing their magnetic properties under strain, which may be measured externally by contactless methods using inductive sensors.
However, these types of measurements are extremely complex and usually not sufficiently precise.
In particular, if these methods are used under operating conditions, there is an array of aspects and problems to be considered. First, the shear modulus of the shaft varies from shaft to shaft in a range of up to a few percent. Furthermore, the shear modulus is a function of temperature: typically it changes 0.025%/° C. This is particularly disadvantageous for the measuring methods, since the shaft changes its temperature by several hundred degrees in the operating range of an engine, for example. However, taking this temperature change into consideration is only possible with difficulty, since the temperature distribution along the shaft is complex. Furthermore, the shaft expands at high speeds due to centrifugal force and thus becomes more rigid. In addition, the torsion of the shaft at full load is relatively small and is typically only a few degrees of angle. The shaft may execute complex radial movements and change its axial position in relation to the engine housing in operation. In addition, the blades of the compressor and the turbine bend axially and around the circumference under load. The engine housing and the housing of a gas turbine are also subject to changes during operation. Thus, the housing may bend and twist due to thermal and mechanical loads. Finally, the use of foil strain gauges has the disadvantage that they barely allow precise static measurements at temperatures above 200° C., since both the strain gauges and also the adhesive begin to creep.
Therefore, because of these boundary conditions of engines and gas turbines, precise torque measurements in the engine or on gas turbines is extremely difficult and very complex using the standard methods.
For measurements of local torsion, multiple strain gauges must be attached in order to be able to separate torsional strains from bending strains which occur. Furthermore, thermocouples are required on the shaft for precise temperature measurement. A telemetry system is required for powering the sensors and for signal transmission. The sensor system and the shaft must be calibrated before the measurements, i.e., defined torques and temperatures must be applied during calibration.
For measurements of the torsion angle, the main difficulties are the deformations of the engine housing or the housing of a gas turbine to which the sensors are ultimately attached, and the temperature distributions in the shaft, which may be applied at sufficient precision during calibration only with difficulty.